To achieve very high fuel utilizations, about 98% or 99%, when operating a fuel cell stack 9 on pure hydrogen, a cascade fuel flow field, illustrated in FIG. 1, comprises a plurality of groups 10-12 of fuel cells 13 arranged in flow-series relationship so that fuel from a source (not shown) passing through a fuel inlet valve 16 enters a fuel inlet manifold 17, flows through a first group 10 of fuel cells 13, then enters a first turn-around manifold 19, then flows through the second group 11 of fuel cells 13, thence through a second turn-around manifold 20 and through the third group 12 of fuel cells 13, to an exit manifold 22.
For a typical 40 kilowatt fuel cell stack, the first group 10 has a large number of cells 13, which may be on the order of about 200 cells, the second group 11 has a lesser number of cells 13, which may be on the order of about 70 cells, and the third group 12 may have on the order of about 25 cells. As is known, this assures that all of the cells get adequate hydrogen even with high hydrogen utilization, provided that the last group of cells 12 get adequate hydrogen.
Referring to FIG. 1, during the production of electricity in normal fuel cell operation mode, a microcontroller 25 provides a signal on a line 26 to cause a fuel inlet valve 16 to be open, to provide fuel to the inlet manifold 17. The processor 25 also provides a signal on a line 27 to cause a normal fuel outlet valve 23 to be open. Under this condition, the fuel enters the inlet manifold 17, passes through the group 10 of cells 13, into the first turn-around manifold 19, through the group 11 of cells 13, through the second turn-around manifold 20, through the group 12 of cells 13, through the exit manifold 22, through the outlet valve 23, and to the exhaust 30.
The fuel cell stack may include a recycle loop 38 driven by a pump 39, all in a conventional fashion; however, the use of a recycle loop is optional.
In U.S. Pat. No. 6,887,599, it is shown that the more rapidly the fresh hydrogen-containing fuel flows through the anode flow field upon start-up, to displace the air therein, the quicker the hydrogen/air interface moves through the anode flow field, and the less time there is for the occurrence of corrosion of the platinum catalyst and catalyst support.
In U.S. Pat. No. 6,821,668, a cascade reactant flow field of a fuel cell stack has additional fuel inlet valves to provide inlet fuel directly to each cascade of the stack and at least one additional exhaust valve to remove fuel directly from each cascade of the stack. This may be used for rapid deployment of fuel into the fuel flow field during start-up.
Although rapid purging reduces startup problems referred to hereinbefore, performance decay of cascaded fuel cell stacks is still unacceptable.
It is also known in the art, as illustrated in the aforementioned U.S. Pat. No. 6,887,599, to connect a voltage limiting device 45 across the main electrical output terminals 46, 47 as soon as a predetermined cell voltage, such as about 0.2 volts per cell is detected by a voltage sensing device 48, which causes the microcontroller 25 to close a switch 50. The voltage limiting device, in the aforementioned application, is simply an auxiliary load resistor.
When it is determined, either by the passage of time or by sensing parameters of the fuel cell stack, that normal operation can be achieved, the micro controller 25 will close a switch 52 to connect the main load 53 across the fuel cell electrical output terminals 46, 47, and open the switch 50.
If the voltage limiting device is not connected across the stack while the groups of cells 10 are being fed hydrogen, then these cells will have excessive voltage, and resulting carbon corrosion and ultimate performance decay. On the other hand, if the voltage limiting device 45 is connected across the stack prior to hydrogen reaching the second and third groups 11, 12 of cells, the anodes in the cells in the second and third groups are driven to an elevated potential that results in corrosion of the carbonaceous catalyst support and other components of the cells.
The foregoing problems have resulted in the conclusion that fuel cell stacks with cascade fuel feed are impractical, due to the certainty of early performance decay.